Many liquid crystal elements are used in display applications. Many liquid crystal elements in display applications realize a gradation display, not a two-grade, black and white display. In addition, various techniques have been developed in pursuing improved viewing angles required for viewing from different angles.
On the other hand, liquid crystal elements in applications other than display include a liquid crystal switching element that transmits the incident light with the polarization plane selectively rotated. In terms of display, such liquid crystal switching elements are elements switching two states, black display and white display. In many cases, they have two different action; they transmit a linearly polarized light with or without the polarized light transmission axis rotated by 90 degrees. In the present specification, liquid crystal elements switching these two states are referred to as polarization switching liquid crystal elements.
Liquid crystal materials and structures different from those in display applications are being studied for the polarization switching liquid crystal element used for switching. For example, one of the requirements for switching elements is the switching speed. Some known elements use ferroelectric liquid crystal for higher switching response speeds. However, ferroelectric liquid crystal is unstable in terms of durability and expensive and therefore not practical.
TN (twist nematic) liquid crystal is known for its stable durability and inexpensiveness. However, TN liquid crystal generally has a lower response speed compared with ferroelectric liquid crystal. Therefore, the thickness of liquid crystal (cell gap) must be small in order to realize a high response speed using TN liquid crystal. However, when the cell gap is simply reduced, the rotation of the polarization becomes wavelength-dependent.
A known means for eliminating the wavelength-dependency consists of, for example, a combination of phase plates (for example, see Patent Document 1, Japanese Patent Application Laid-Open No. H7-64048). In such a case, the addition of phase plates increases parts cost.
Some image display apparatuses having a spatial light modulation element such as a transmissive LCD utilize the wobbling technique in which a pixel shifting element is used to improve the resolution of the spatial light modulation element. In the wobbling technique, a pixel shifting element consisting of a polarization switching liquid crystal element for selectively rotating the polarization by 90 degrees and a birefringent plate for selectively shifting the light beam according to the polarization direction is used to synchronize the pixel shifting due to the light beam shifting with the image on the spatial light modulation element corresponding to the shifted light beam position for higher resolution.
It is advantageous that the polarization switching liquid crystal element used in the above pixel shifting element is capable of high speed response. Therefore, ferroelectric liquid crystal is used in some known cases (for example, see Patent Document 1). However, ferroelectric liquid crystal is unstable in terms of durability and expensive as described above and is therefore not practical.
In some known cases, durable and inexpensive TN crystal is used and the drive timing is optimized in consideration of the response speed (for example, see Patent Document 2, Japanese Patent Application Laid-Open No. H11-296135). However, this technique has limitations in applying to field-sequential image display apparatuses in which R, G, and B colors are sequentially displayed. There is also a demand for TN liquid crystal having a high response speed.
Field-sequential image display apparatuses (for example projectors) have been realized as color field-sequential image display apparatuses as the spatial light modulation element has higher response speeds. In the color field-sequential image display apparatus, one spatial light modulation element is illuminated with R, G, or B color light in sequence to modulate them according to information of each color for color display. In this way, a simplified and inexpensive structure can be realized compared to three-plate image display apparatuses in which three spatial light modulation elements are used for R, G, and B colors.
The color field-sequential image display apparatus has color breaking as a result of sequential color illumination. Therefore, higher display frequencies are required for the respective colors. When the pixel shifting is performed by wobbling in the color field-sequential image display apparatus, the polarization switching liquid crystal element consisting of TN liquid crystal has to be capable of high speed operation corresponding to the spatial light modulation element.
The high speed response capability of the polarization switching liquid crystal element can be improved to some extent by optimizing the drive timing of the TN liquid crystal. In order to improve the wobbling performance, TN liquid crystal having a higher response speed is desired.
For example, when the cell gap is reduced in order to increase the response speed of TN liquid crystal, false colors appear because of the wavelength-dependent light rotation property, deteriorating the displayed image quality. In order to prevent false colors due to the wavelength-dependency, phase plates can be provided as disclosed in the above patent Document 1. However, parts cost may be increased as described above.